January 30, 2013

Interaction Between Solar Wind And Venus Measured By Venus Express

At the very edges of the Earth´s atmosphere, where the Earth´s magnetic field begins, solar radiation strips electrons from the tenuous gas that is slowly leaking into space.

This region, known as the ionosphere is famously responsible for allowing terrestrial radio transmissions over great distances. During World War II, for example, radio operators would “bounce” signals off the ionosphere to send messages to forces half way around the world. (Keeping in mind this was before the age of satellites.)

While the amount of ionization in this region is dependent on the mood of the Sun — how much solar wind is streaming towards the Earth at any given time — the general shape and geometry of the ionosphere is relatively stable. This is primarily due to the fact that Earth has a strong, internally generated magnetic field.

What has not been clear, however, is what happens when very little magnetic field is present to contain the ionization field. To explore this question, scientists turned to the planet Venus.

This nearby world is similar to Earth in relative size and structure, yet has a much thicker atmosphere - making it most inhospitable — with virtually no internally generated magnetic field.

Previous studies of Venus´s ionosphere showed a region that was relatively stable, unperturbed by the small variations in solar activity. However, the scientific community lacked a consensus as to how extremely low solar activity would affect the shape of the ionization region.

Then, for an 18 hour period in August 2010, solar wind density dropped to 0.1 particles per cubic centimeter, or roughly one fiftieth the normal level. The European Space Agency´s (ESA´s) Venus Express seized the opportunity and measured an increased rate of ionized particles forming a teardrop shape, reminiscent of the dust and gas tails created by comets passing through the inner solar system.

Researchers concluded that the ionosphere expands significantly when the pressure from the solar wind drops. According to HÃ¥kan Svedhem, ESA´s Venus Express project scientist, “We often talk about the effects of solar wind interaction with planetary atmospheres during periods of intense solar activity, but Venus Express has shown us that even when there is a reduced solar wind, the Sun can still significantly influence the environment of our planetary neighbors.”

Researchers have published their findings in the Planetary and Space Science journal.